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Geology

Rainfall is higher and the wind changes direction in north Australia during the summer monsoon season. The history of the Australian monsoon is not clear because of Australia's poor climate record. However, fossil remains of animals and plants suggest that central and north Australia was warm, wet, and possibly seasonal during the early to middle Miocene - a time of unexplained global warmth about 15 million years ago. Nicholas Herold of the University of Sydney and colleagues use computer models and available climate records to look at the size and strength of the summer monsoon in the Miocene. They show that the monsoon in the Miocene was probably weaker than the monsoon of today, suggesting that large amounts of carbon dioxide in the atmosphere allowed Miocene plant life to exist, and was a major cause of Miocene global warming.

***************Low marine sulfate concentrations and the isolation of the European Epicontinental Sea during the Early Jurassic
Robert J. Newton et al., School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK.
Pages 7-10.

It is surprisingly difficult to produce estimates of even the main components of seawater chemistry in the past. In the modern ocean, the isotopic composition of seawater sulfate is near constant regardless of where you take your sample, but new research shows that this was not always the case. Robert J. Newton of the University of Leeds and colleagues analyzed samples of Early Jurassic carbonate rocks and fossils from two sites for a proxy for sulfate isotope composition -- one from a wide, shallow sea centered in Europe and the other from a distant site much closer to deep water. They found very large gradients in sulfate isotope composition which can only be explained if ocean sulfate concentrations were about one sixth that of the present day, substantially below previous estimates for the time period. The other implication is that water only circulated slowly between the shallow sea and deep ocean, allowing the shallow sea to develop its own separate chemistry. This is an important consideration because records of ocean chemistry prior to the Cretaceous are mainly derived from seas of this type due to the subduction and destruction of deep ocean crust.

Characterizing how interactions between the solid Earth, the atmosphere, oceans, and biochemical cycles might modulate global climate is a primary target in earth system research. The early Miocene (circa 23-16 million years before present) was a time of rapid deformation, uplift, and erosion in the High Himalayas. Previous models have correlated these events with the onset of the Asian monsoon, and suggested that it contributed ultimately to mid-Miocene global cooling via increased weathering and/or organic carbon burial, and consequent atmospheric carbon dioxide drawdown (the negative greenhouse effect). Howard A. Armstrong and Mark B. Allen of Durham University compare the position of the Intertropical Convergence Zone (ITCZ) with the paleolatitude of the Himalayas, and show that these converged in the early Miocene. Because the ITCZ is a zone of high precipitation, they suggest that this convergence was an important driver of early Miocene High Himalayan erosion. Continued northward motion of the Indian plate took the Himalayas north of the ITCZ, which migrated south through the remainder of the Miocene. High Himalayan erosion correspondingly decreased after the early Miocene. This new mechanism for increased Himalayan erosion is independent of the timing of the onset or enhancement of the Asian monsoon, which is much debated.

Geochemical analysis by Mark Clementz of the University of Wyoming and colleagues of early Eocene sediments from a locality in Vastan, India, provides new constraints on the timing of collision and the exchange of terrestrial faunas between Asia and the Indian subcontinent. Strontium isotope analysis of marine fossils and carbon isotope analysis of terrestrial organic material support an age of circa 54.0 million years before present for these deposits, which coincides with the second hyperthermal event following the Paleocene-Eocene Thermal Maximum (PETM). Based on organic delta-13C values for this section, the Vastan sediments provide the first record of terrestrial organic matter from a low-latitude location to show clear, isotopic representation of this hyperthermal event. Mammal fossils recovered from below this event are the oldest Cenozoic mammal fossils recovered so far from India, and indicate an earlier occurrence of modern orders of placental mammals in India than previously thought. The occurrence of a high number of these terrestrial mammals in India at this time also provides a minimum estimate of the timing of subaerial contact (circa 54.0 million years before present) between the Indian subcontinent and Asia, implying that initiation of the collision must have occurred even earlier.

Brian M. Chase of the University of Bergen and colleagues detail the discovery of sensitive indicators of past climate change in the stratified fecal deposits (known as middens) of rock hyraxes in the Cederberg Mountains of southernmost Africa. As a new palaeoenvironmental archive, rock hyrax middens offer the first opportunity to obtain reliable high-resolution records that can provide detailed information regarding past climate change in the region. In comparison with other terrestrial palaeoenvironmental archives from the region, which can generally only be resolved to millennial to multi-millennial time scales, hyrax middens provide an improvement in resolution of three to four orders of magnitude. The records Chase et al. have obtained provide the first opportunity to precisely identify the nature and timing of terrestrial environmental change in the study region during the Last Glacial-Interglacial Transition (18-11.5 thousand years before present), revealing the first unequivocal terrestrial manifestation of the Younger Dryas (an abrupt return to near-glacial conditions that occurred between 13-11.5 thousand years before present) from the southern African subtropics. These results provide key evidence regarding the processes and feedbacks within the global climate system, and suggest that changes in Northern Hemisphere climate have exerted a strong influence on all but the higher latitudes of the Southern Hemisphere since 15 thousand years before present.

Most of the snow that falls on the Antarctic Ice Sheet drains to the sea through narrow, dynamic ice streams. Theoretical and numerical modeling, along with observations of modern ice streams, have indicated that they are highly sensitive to climate and sea-level fluctuations, but the models that project the future response of the East Antarctic Ice Sheet have not included this sensitivity. White et al. investigated the response of one of East Antarctica's largest ice streams to climate and sea-level fluctuations at the end of the last ice age by dating the moraines and sediments left by the retreating ice stream. They determined that the ice stream began to retreat many thousands of years earlier than the surrounding ice sheet, and that there was limited delay between regional climate warming and lowering of the ice stream several hundred kilometers inland. These results suggest that the East Antarctic Ice Sheet may be more sensitive to climate warming and sea-level rise than existing projections suggest, providing further urgency for the inclusion of ice streaming into future global earth system models.

This study by Ryan Crow of the University of New Mexico and colleagues uses the age and composition of basaltic lava flows from around the Colorado Plateau to track how the North America plate is being uplifted and modified by deep mantle flow. New ages on western Grand Canyon basalts and a compilation of all the dated basalts from the region show that magmatism has migrated closer to the plateau center through time. This trend is seen all around the plateau margins, but the rate of encroachment toward the plateau center is fastest around the southern margins, where it averages 5-8 mm/year. Basalts also change isotopic composition with age and position, with younger basalts being more asthenospheric, meaning that the youngest basalts are dominantly sourced from the flowing asthenosphere that resides below the North America plate. The data indicate that during the last 25 million years, mantle has been upwelling under the margins of the Colorado Plateau, converting or eroding away the base of the plate. These data provide convincing petrologic evidence that asthenospheric upwelling has been ongoing for the past several million years, and is likely causing uplift of the Colorado Plateau margins.

Between 600 and 700 million year ago, Earth froze solid in an event dubbed Snowball Earth. How life survived this deep freeze has puzzled scientists for over a decade. Now geologists at Royal Holloway, University of London may have found the answer. Daniel Paul Le Heron and colleagues have uncovered evidence for large "ice-free oases" that could have harbored life and helped animals beat the chill. Exploring the remote central Flinders Ranges in South Australia, they have found structures in rocks laid down about 700 million years ago that could only have been formed by storms on the oceans' surfaces. These could only have formed if there was no ice. While Australia was near the equator during the Snowball Earth, a 5-km-thick pile of debris was laid down by ice sheets at the equator. The newly discovered structures, known to form only by storm activity, occur in the middle of the debris and point clearly to ice-free conditions. Le Heron says, "This is an incredibly exciting discovery. What we've found is the clearest evidence yet that large areas of the Earth's oceans remained ice-free during Snowball Earth. Such 'oases' would have been key for the survival of life ... and indeed our own existence today."

Nicola De Paola of the University of Durham and colleagues performed laboratory friction experiments on thermally unstable rocks (e.g., carbonates) under conditions similar to those expected during the propagation of large earthquakes (i.e., for greater than 1 m/s slip velocities, and temperature rise on the order of few hundreds of degrees). Their results show that fault weakening occurs during the propagation of a seismic event in thermally unstable rocks, because a dynamic fault strength drop up to 90% was measured. Fault lubrication is controlled by the superimposition of multiple, thermally activated, slip-weakening mechanisms (e.g., thermal pressurization and nanoparticle lubrication), which are still poorly understood due to the scarcity of experimental evidence. Seismic source parameters calculated from De Paola et al.'s experimental results match those obtained by modeling of seismological data from the 1997 Cofliorito earthquake nucleated in carbonate rocks in Italy (i.e., the same rocks used in the friction experiments), yielding further soundness about the extrapolation of the experimental results to seismic rupture propagation in the upper crust under natural conditions. The experimental results improve the understanding of the controls exerted on the dynamic frictional strength of faults by the coseismic operation of chemical (mineral composition) and physical (grain size, fluid content, etc.) processes.

Ancient magnetic directions trapped in rocks are consistent across the Australian continent during the past 500 million years, but in rocks formed before 650 million years ago (Ma), the magnetic directions from northern Australia are offset from those of southern and western Australia by an angle of about 40 degrees. The directions become realigned if the two parts of Australia are rotated relative to each other by the same angle, thus producing a modified map of the continent prior to its final assembly. The interpretation presented here by Zheng-Xiang Li and David A. D. Evan of Curtin University resolves a longstanding enigma of when the supercontinent Rodinia broke apart, thus allowing for a tighter-fitting Rodinia to last until at least 720 Ma, which was right before the hypothesized first late Precambrian Snowball Earth ice age. The 40 degree rotation within Australia between 650 and 550 Ma occurred during the breakup of the supercontinent Rodinia and the assembly of the Gondwana supercontinent, causing the formation of a Himalayan-style mountain belt across the center of Australia. The sandstone and conglomerate rocks of Uluru (Ayers Rock) and Kata Tjuta (The Olgas), respectively, were deposited in the foothills of that now-eroded mountain range.

Is mountain-building a source of or sink for carbon dioxide? It is widely accepted that weathering of common rock-forming minerals, particularly in young mountains such as the Himalaya, removes carbon dioxide from the atmosphere. It is also known that metamorphic reactions, caused by mountain building, release carbon dioxide. This "metamorphic carbon dioxide" moves upward toward Earth's surface, where its escape to the atmosphere is detectable in mountain hot springs. Alasdair Skelton of Stockholm University presents the results of a novel approach to measuring the rate at which metamorphic carbon dioxide migrates to Earth's surface. The partial replacement of layers of volcanic rock from an ancient mountain belt in Scotland by carbon dioxide-bearing minerals is used to estimate metamorphic carbon dioxide flux rates, providing an analogue for the same process which is ongoing in younger mountain belts today. Skelton’s results show that the rate at which metamorphic carbon dioxide migrates to Earth's surface exceeds the rate at which carbon dioxide is removed from the atmosphere by weathering but matches the rate at which carbon dioxide escapes from hot springs. This suggests that mountain building is a net source (not sink) of carbon dioxide.

Backarc basins develop behind arc-trench systems where the arc crust is ruptured and new oceanic crust is formed. Several backarc regions active today have experienced multiple, episodic phases of opening, and the magmatic and tectonic mechanisms that drive these events are still a matter of debate. The Godzilla megamullion is the largest known oceanic core complex (OCC). It is situated adjacent to the Parece Vela Basin (PVB) spreading center, an extinct backarc basin in the Philippine Sea. OCCs are dome structures adjacent to spreading centers that expose gabbros and serpentinized peridotites from deep in the oceanic lithosphere. The timing of the Godzilla megamullion formation was poorly constrained due to weak geomagnetic anomalies in the region. New zircon U-Pb ages obtained by Kenichiro Tani of the Japan Agency for Marine-Earth Science and Technology and colleagues reveal that fault-induced spreading formed the Godzilla megamullion over 4 million years, and that the spreading extended up to 125 km in length, with continuous magmatic accretion at the spreading axis. The zircon ages constrain the cessation of spreading in the PVB to be less than 7.9 million years ago, significantly younger than the previous geomagnetic estimate of 12 million years ago. The new ages show that backarc basin formation migrated to the present-day Mariana Trough soon after spreading ceased in the PVB.

***************Shallow-water carbonate record of the Paleocene-Eocene Thermal Maximum from a Pacific Ocean guyotStuart A. Robinson, Dept. of Earth Sciences, University College London, Gower Street, London W1CE 6BT, UK. Pages 51-54.

Anthropogenic carbon dioxide emissions are causing changes in surface ocean temperatures and chemistry that may be detrimental to the sustainability of shallow-water carbonate producers and environments (such as coral reefs, carbonate banks, and lagoons). The geological record can provide constraints on the response of these ecosystems to past carbon cycle perturbations. The Paleocene-Eocene Thermal Maximum (PETM; circa 55.8 million years ago) was a geologically brief interval of global warming coincident with a large input of carbon that can be recognized in the rock record by a change in the ratio of the two stable isotopes of carbon, known as 12C and 13C. This study by Stuart Robinson of University College London recognizes for the first time this characteristic carbon-isotope "signature" of the PETM in shallow-marine limestones from an open ocean setting. The sediments were originally deposited in a shallow lagoon on top of a volcanic seamount in the tropical Pacific Ocean and show no major evidence for a carbonate production crisis during the PETM, suggesting that the effects of changes in temperatures or surface ocean pH were relatively short lived or relatively minor. The PETM likely provides only a minimum estimate of the response of marine calcifiers to future surface-water environmental change.

Levee failures have occurred along the Mississippi River corridor throughout the river's ~300-year history of flood-control efforts. Most recently, levee failures along the Upper Mississippi River in 2008 and breaches in the New Orleans flood-protection system in 2005 have made renewed levee safety a national priority. In this study by Andrew Flor, Nicholas Pinter, and Jonathan W.F. Remo of Southern Illinois University-Carbondale, levee-crest elevations were precisely measured (with geodetic-grade GPS) over a 9-year time span along 328 km of Mississippi River levees south of St. Louis, Missouri. The majority of surveyed levees were stable, but 35-59 km were distinguishably higher (up to 1.49 m higher) and 20-55 km were distinguishably lower (max. decrease of 1.26 m). Elevation decreases are interpreted as degradation of levee fill and/or compaction of the underlying soils. Elevation increases include local crevasse repairs as well as broader levee-raising projects, both permitted and unpermitted. Levee degradation reduces protection levels for floodplain residents, often without easily visible symptoms. Intentional levee raising has a range of potential negative impacts, including a false sense of security and the potential for exporting flood risk to neighboring levee communities. Regional surveying of levee elevations and change over time can provide a preliminary tool for assessing levee conditions, stability, and compliance with levee regulations.

During an earthquake, fault motion can be as fast as a few meters per second. Considering the high stresses acting on faults, significant frictional heating - and thus temperature increase - is expected when an earthquake occurs. By performing high-velocity friction tests on gypsum cylinders to simulate such a fast motion, Nicolas Brantut of the Ecole Normale Superieure, Paris, and colleagues bring evidence that the temperature increase due to frictional heating can be significantly limited by rapid metamorphic dehydration reactions. Indeed, data analyses indicate that 10%-50% of frictional heating is actually turned into latent heat of reaction in spite of temperature increase. The thermal behavior of active faults can thus be strongly affected by such fast metamorphic reactions. Field evidence of this "coseismic" metamorphism could be left over in the rocks, and may be used as an indicator of ancient earthquakes.

***************
Changes in the Indonesian Throughflow during the past 2000 yr
Alicia Newton et al., Dept. of Earth and Ocean Sciences, University of South Carolina, Columbia, South Carolina 29208, USA. Pages 63-66.

In the tropics, water flows from the Pacific Ocean into the Indian Ocean through the Indonesian Throughflow, a current system that passes through the islands of Indonesia. The temperature of the water carried by the Indonesian Throughflow affects the subsurface temperatures of the Indian Ocean in the tropics. Alicia Newton of the University of Carolina and colleagues used marine sediments to reconstruct the flow of surface water through a key point in the Indonesian Throughflow - the Makassar Strait - for the past 2,000 years. They find that the amount of surface water transported varied throughout that time, with prolonged periods of increased surface flow associated with La Niña-dominated conditions.

The water content and viscosity of magmas are major controls on the explosivity of volcanic systems. Highly viscous and water-rich silicic magmas form the most explosive volcanic systems on Earth. This is in part because water dissolved in melt exsolves into bubbles that are unable to escape from viscous magmas. Accumulated pressure in bubbles can be key triggers in explosive eruptions, and vesicles in pumice are records of these bubbles. However, silicic magmas also erupt as lavas, forming non-vesicular glass known as obsidian. Worldwide, such obsidian flows are typically water-poor despite their original magmas being water-rich. How did obsidian lose its water? Agustin Cabrera of Monash University and colleagues investigate water dissolved in an obsidian block that has been faulted and glued back together again while still liquid in the volcanic conduit, before being erupted. Results indicate that cracking of viscous melt simultaneously created a low-pressure site through which water was extracted from the surrounding melt, and a pathway for water to exit the system as a whole. In this manner, faulting provided a mechanism of melt dewatering. They argue that water loss through intense fracturing could explain the origin of water-poor obsidian, and most significantly, it helps defuse explosive volcanic systems by allowing water escape.

The role of mountain building in moderating Earth's carbon cycle and climate is of long-standing debate. It is commonly assumed that the process is associated with a return of carbon dioxide to the atmosphere due to the complete chemical alteration of fossil organic carbon, sequestered in the geological past and exposed within sedimentary rocks brought to the surface. Combining geochemical characterization of river sediments with measurements of water discharge and sediment load, Robert G. Hilton of the Institut de Physique du Globe de Paris and colleagues have quantified the export of fossil organic carbon from the small but rapidly eroding mountain island of Taiwan. They find that, in contrast to larger systems such as the Amazon River, the vast majority (more than 85%) of fossil organic carbon in steep mountain rivers in Taiwan remains locked in sediments and are supplied to the ocean. Export of fossil organic carbon from this island represents 1%-2% of the global organic carbon supply to the ocean. Indeed, its reburial in marine sediments may match or exceed the reburial of fossil organic carbon eroded from the Himalaya and deposited in the Bengal fan. This work supports the notion that ocean islands and coastal mountain ranges may be disproportionately important for the long-term retention of organic carbon in the lithosphere.

Workers from the University of Bergen, Norway, demonstrate how magnetotactic bacteria (MTB), which are among the oldest prokaryotes found in the fossil record, colonized four freshwater lakes located along a north-south transect in Norway some time after the retreat of the last Fennoscandian Ice Sheet. Perhaps most remarkable about this discovery is that the colonization was synchronous, taking place 9,760 years ago (plus or minus 160 years). Although it is beyond the current precision of radiocarbon dating, it is possible that this synchronized post-glacial colonization of MTB actually happened within decades. The lakes from which these reconstructions are produced are situated more than 1400 km apart, representing both coastal and inland regions, and with altitudinal differences of almost 800 m. Regardless of environmental differences between the lakes, the colonization without exception lags the glacial-interglacial transition by about 2000 years. Paasche and Løvlie propose that birds could have carried and spread the bacteria, as northward migration routes were re-established following the onset of the current interglacial. This unique data set underscores the tenacity of MTB as evolutionary survivors, and also demonstrates their response to large-scale environmental changes in ways not previously anticipated.

Idael Francisco Blanco-Quintero of the Universidad de Granada, Spain, and colleagues present the first petrological description of convective circulation of the materials in the Caribbean subduction channel, previously predicted by numerical modeling. The block appears in La Corea mélange (eastern Cuba), a piece of the subduction mélanges in the north Caribbean region that constitutes one of the most spectacular subduction channels exhumed in the world. Garnet porphyroblasts show complex growth/dissolution processes during the rock evolution. Pressure-temperature conditions determined using the chemical zoning and mineral inclusions in garnet porphyroblasts show recurrences; these recurrences were interpreted as the consequence of convective circulation in the subduction channel.

There has been a long-standing debate on whether the ability of a fault to generate an earthquake depends on how strong the fault is. It has been suspected that strong faults might be more likely to have earthquakes, but there is no experimental evidence to support this. Matt J. Ikari of Pennsylvania State University and colleagues present evidence that, for a variety of rock-forming minerals covering a spectrum of weak to strong, only strong fault material can generate earthquakes. Additionally, the fault must have also slipped a certain distance in order for earthquakes to occur.

In order to understand the linked tectonic and climatic histories of mountain belts, it is essential to determine how and when they reached their peak heights. Nowhere is this information more critical than in the North American Cordillera where its surface uplift history is unknown. Hari T. Mix of Stanford University and colleagues used the isotopic signature of ancient precipitation preserved in minerals to reconstruct the elevation history of western North America. To do this, they used the well-established relationship between the isotope composition of rainwater and elevation that results from rainout as air masses pass over mountain ranges. With a data set of nearly 4,000 samples, the authors noted a prominent southward sweep of high elevations that began about 50 million years ago in southern Canada and reached central Nevada by 39 million years ago. This topographic wave is coincident with prominent volcanism and the initiation of extension in the Cordillera, which also migrate from north to south. Together these observations demonstrate the north-to-south removal of the ancient Farallon slab as instrumental in the formation of the North American Cordillera.

Much of the geological record on Earth can be interpreted in the context of active processes occurring at the plate boundaries. For Phanerozoic (younger than 570 million years) rocks this is well established, but during the Precambrian (older than 570 million years), when the oldest rocks were forming, Earth conditions were likely very different, so analogies to modern-day tectonics are less certain. For example, 40 years after the advent of plate tectonic theory, the precise onset of continental drift remains ambiguous: In the past five years, its onset has been estimated as early as about 4.1 billion years ago, or as late as about 1 billion years ago. Canada is home to rocks that span much of early Earth's history, and data from new seismograph networks in this remote and ancient region provide fresh scope to address this problem. Using seismology, Ian D. Bastow of the University of Bristol and colleagues map the boundary between two ancient plates that collided during the Trans-Hudson Orogen. Their results support the view that Himalayan-scale plate deformation affected much of northern Canada and that modern-day plate tectonic processes were in operation by at least 1.8 billion years ago.

Some of the oldest colonies of organisms on Earth are found in some of the most unexpected, extreme, environments. Tim K. Lowenstein, Brian A. Schubert, and Michael M. Timofeeff of the State University of New York-Binghamton studied tiny parcels of fluid inside halite crystals (fluid inclusions) in modern and ancient buried salt crystals from Death Valley and Saline Valley, California. There they discovered an ecosystem of “salt-loving” (halophilic) single-celled organisms -prokaryotes and eukaryotes - some of which are alive despite the antiquity of the crystals in which they reside. Research has demonstrated that prokaryotes may survive inside fluid inclusions for tens of thousands of years using carbon and other metabolic products supplied by the trapped microscopic community, most notably a single celled alga Dunaliella. This creature is coming to be recognized as an important base-level constituent of food chains in hypersaline systems. Deeper understanding of the long-term survival of prokaryotes in fluid inclusions will complement studies that further explore microbial life on Earth but, even more exciting, potentially beyond Earth, where materials that could harbor microorganisms are millions and even billions of years old.